Gravitational-wave observations have revealed the population of stellar remnants from a new angle. Yet their stellar progenitors remain uncertain, in particular in the case of black holes. At least some of these progenitors are believed to be formed in isolated binary systems. In this talk, I will discuss how binary mass transfer affects the late evolution and final fate of massive stars. The focus will be on the properties of stars that transfer their outer layers to a companion star and become binary stripped. I will show that binary-stripped stars develop different core structures from single stars and discuss consequences for supernovae progenitors, black hole formation, nucleosynthesis, and gravitational-wave sources.